The present disclosure relates to a method of fabricating high resolution Atomic Force Microscopy (AFM) tips and, more particularly to a method of fabricating high resolution AFM tips that have a controllable diameter and a high aspect ratio.
Atomic force microscopy (AFM) is a very high resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, which is more than 1000× better than the optical diffraction limit. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale level. The information is gathered by “sensing” the surface with a mechanical probe or tip. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable very precise scanning. In some variations, using conducting cantilevers is necessary. In newer more advanced AFMs, current can even be passed through the tip to probe the electrical conductivity or transport of the underlying surface.
The AFM consists of a cantilever with a sharp tip at its end that is used to scan a sample surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, Casimir forces, solvation forces, etc. Along with force, additional quantities such as, electrical potential, conductivity, and/or transport, may simultaneously be measured through the use of specialized types of probes.
Conventional AFM tips are fabricated by anisotropic etching of silicon, Si. These tips are formed at the end of a silicon cantilever and typically have a shape of a pyramid with triangular sides defined by Si (111) surfaces. The drawback of such prior art AFM tips is their poor lateral imaging resolution due to their low aspect ratio and large radius of curvature of about 5 nm-30 nm (best case).
When using AFM to image a property of the surface, the resolution is strongly dependent on the diameter and aspect ratio of the tip. In some instances, a high aspect ratio AFM tip is important when imaging steeply rising steps or narrow trenches. A high aspect ratio is also important when the forced sensed is a long range force. When the force is a long range force, the force decays as a/rn wherein “a” is a constant, n is a positive integer and r is the distance of the tip to the surface. In such a case, not just the atoms at the apex contribute to sensing but the whole tip contributes to the interaction with the surface. Also, in such a case, the higher the aspect ratio and the smaller the tip radius, the better the resolution. The current solutions to this problem including forming nano-materials such as carbon nanotubes or semiconductor nanowires at the end of the Si tip have limited aspect ratios, are not production scale processes, require more difficult lithographic techniques, or have limited control as to the placement and morphology of the tip enhancements.
As such, a method is needed that is capable of fabricating high resolution AFM tips that have a controllable diameter as well as a high aspect ratio. Also, there is a need for providing high resolution AFM tips that have little or no tapering associated therewith.